This invention relates to a refrigeration system and more particularly to a refrigeration system including a hermetic motor-driven compressor that is exposed to outdoor ambient temperatures with certain other components of the system.
It is desirable that the compressor in a refrigeration system be lubricated with oil-refrigerant solution rich in oil at all times during start up and stable running conditions. Because of increased oil temperatures during running, the compressor lubricating oil will contain in solution only a small amount of refrigerant. However, during off periods the compressor and lubricating oil will come to ambient temperature following a diurnal cycle which will lag the other system component temperatures because of its mass. The vapor pressure of the oil/refrigerant solution in the compressor sump is less than the vapor pressure of the remaining pure refrigerant at equal system temperatures and considerably less when the compressor is at a lower temperature than the other components.
During these diurnal temperature cycles, refrigerant present in the system as a liquid will migrate by gaseous transfer to the oil in the compressor sump. If the system is allowed to remain off for a long enough period, all the liquid refrigerant will migrate to the compressor. The lubricating oil will continue to absorb refrigerant and two-phasing, i.e. oil rich/liquid refrigerant rich separation, will occur. The refrigerant will continue to migrate into the compressor and collect as refrigerant rich liquid in the bottom of the sump (refrigerant heavier than oil) until all the liquid refrigerant in the remaining system has been transferred to the compressor. If the compressor is started during the time when extreme oil dilution and/or two-phasing is present in the sump, bearing damage and subsequent compressor failure can occur.
The most common prior art means of preventing two-phasing of refrigerant and oil or extreme oil dilution from occuring is to provide heat to the oil sump generally using an external-type resistance heater attached to the compressor case in the sump area as shown in U.S. Pat. Nos. 2,175,913-Philipp and 3,577,741-Shaw. The purpose of the heater as is known in the art is conventionally to attempt to keep the compressor warm enough to avoid extreme oil dilution and the collection of liquid refrigerant in the compressor housing. These heaters when employed must have enough wattage to raise the sump temperatures high enough to maintain the sump temperature generally higher than the remaining outdoor component temperatures. This method of two-phase control reduces the system EER thus increasing energy usage and general operating costs.
Besides the problem of two-phasing, the presence of liquid refrigerant anywhere in the system can present a problem during initial start up of the compressor. Since the compressor is designed to compress refrigerant in its gaseous state any liquid refrigerant that may be injected into the compressor at start up can cause both bearing and valve damage.